1. Field of the Invention
The present invention relates to an ink-jet head driving method and an ink-jet recording apparatus in which an ink drop is ejected from a nozzle by varying the capacity of a pressure chamber that contains ink.
2. Description of the Related Art
FIG. 11 illustrates a configuration of a conventional ink-jet recording head. In FIG. 11, reference numeral 1 indicates an ink-jet recording head. The ink-jet recording head 1 includes a plurality of pressure generating chambers 2 to be filled with ink, a nozzle plate 3 provided at one end of each of the pressure generating chambers 2, a nozzle 5 provided in each of the pressure generating chambers 2 to eject an ink drop 4, a piezoelectric actuator 7 for giving vibration to the pressure generating chambers 2 through a vibrating plate 6 and ejecting ink from the nozzle 5 by varying the capacity of the pressure generating chambers 2 with the vibration, and an ink chamber 9 that communicates with each of the pressure generating chambers 2 to supply ink to the pressure generating chambers 2 from a tank (not shown) through an ink supply path 8.
With the above configuration, when the piezo-electric actuator 7 is driven, the pressure generating chambers 2 are vibrated. This vibration varies the capacity of the chambers 2 to eject an ink drop 4 from the nozzle 5. The ink drop 4 reaches a recording medium such as recording paper and forms a dot thereon. If such dots are formed in sequence, given characters, images, etc., which correspond to image data, are printed on the recording medium.
In the ink-jet recording head 1 described above, an ink drop needs ejecting with stability to correctly print characters and images on a recording medium based on input printing information.
However, the actual use of the ink-jet recording head 1 for printing may cause a problem in which an ink drop is ejected unstably due to various factors and thus a desired printing result cannot be obtained. One of the factors is evaporation of volatile components from ink.
More specifically, ink used for ink-jet recording employs water as the main solvent, and coloring such as various kinds of organic solvent dye such as a surface-active agent is added to the water. If no ink drops for some long period of time, moisture is drawn from an opening of the nozzle 5 that is exposed to outside air. The ink therefore increases in viscosity or partly solidifies to block the nozzle 5.
The above problem is resolved as follows. The ink-jet recording head 1 moves away from a printing area and ink is discharged from the ink chamber 9, or ink is discharged from the nozzle 5 by forcibly sucking new ink through the nozzle 5 by means of a pump.
In order to eject ink from the nozzle 5 for high-quality printing with stability, however, the above operation has to be performed frequently. This causes the following problem. An amount of ink consumed increases and so do printing costs, and a large amount of ejected ink should be disposed of.
As a method of resolving the above problem, for example, Jpn. Pat. Appln. KOKAI Publications Nos. 57-61576 and 9-29996 disclose an operation of providing a pressure generating chamber with such a small vibration that no ink drops jump out of the nozzle even when no ink drops are ejected from the nozzle (this operation is called a precursor).
There now follows an explanation as to the precursor referring to FIGS. 12A to 12E. The figures are enlarged views of a nozzle portion of the ink-jet recording head 1. Ink 11 in the pressure generating chamber 2 is exposed to outside air at a portion 13 of the opening 12 of the nozzle 5 as illustrated in FIG. 12A. In the portion 13, as shown in FIG. 12B, moisture is drawn from the ink 11 to form a high viscosity ink layer 14 near the meniscus. If a precursor is carried out as shown in FIGS. 12C and 12D, the meniscus vibration very slightly. With this vibration, the high viscosity ink layer 14 and low viscosity ink layer 23 are agitated to uniform the viscosity of ink in the pressure generating chamber 2 as illustrated in FIG. 12E. In FIG. 12E, reference numeral 15 denotes ink whose viscosity is uniformed.
In order to perform the precursor, a driving voltage that is lower than that for ejecting a normal ink drop has to be applied. Another driving power supply is required accordingly.
Although the above operation (precursor) is effective if no ink drop is ejected for a short period of time, it simply decreases the speed at which the viscosity of ink increases because the ink 11 in the nozzle 5 is not replaced with a new one. If, therefore, no ink drop is ejected for a long period of time, the ink 11 will solidify in the nozzle 5, which makes it difficult or impossible to eject an ink drop again.
When the very small vibration changes the meniscus from a convex to a concave as shown in FIGS. 13B to 13D, ink 11a that increases in viscosity is likely to attach and remain on the nozzle plate 3 near the nozzle. The ink remaining on the nozzle plate 3 causes the ink ejecting direction to be shifted.
For example, Jpn. Pat. Appln. KOKAI Publication No. 9-29996 described above discloses a method including a step (precursor) of providing such a small vibration that no ink drops jump out of the nozzle even when no ink drops are ejected from the nozzle and a step of retreating the ink-jet recording head from a printing area in a fixed period of time and ejecting the ink 11 from the pressure generating chamber 2 and from near the opening of the nozzle 5 (hereinafter referred to as a spit operation). The spit operation requires its own driving voltage waveform whose potential difference is greater than that of a driving voltage waveform used for normal printing, and a large amount of ink 11 is ejected from the pressure generating chamber 2 and replaced with a new one, thereby preventing ink from solidifying and increasing in viscosity for a long period of time.
The method of the Publication necessitates a driving waveform exclusively for the spit operation, and the driving waveform requires three different waveforms of a normal ejecting waveform, a precursor driving waveform and a spit driving waveform. The number of driving power supplies therefore increases to make a driving circuit complicated and thus make the ink-jet recording apparatus expensive.
If the ink-jet recording apparatus turns off and sits idle for a long period of time without performing any precursor or spit operation, the ink 11 remaining near the nozzle 5 increases in viscosity and easily solidifies.
In an ink ejecting operation prior to a printing operation, too, ink that increases in viscosity is attached to the periphery of the nozzle 5 of the nozzle plate 3, as is a coagulation of solidified ink, thereby shifting the ink ejecting direction.